1. Field of the Invention
The present invention relates to laser amplifiers, and in particular, a laser amplifier with variable and matched wavelength pumping of the gain medium disposed in a laser used for laser shock processing.
2. Description of the Related Art
Laser shock processing involves directing a pulse of coherent energy to a piece of solid material to produce shockwaves therein. The produced shockwave causes compressive residual stresses to form within the solid material. These compressive residual stresses improve the fatigue hardness and corrosion resistance properties of the solid material.
The gain medium is composed of atoms having various energy levels. When the gain medium is pumped with optical energy, electrons from the atoms of the gain medium are excited from a ground energy state to excited energy states. This difference is called the pump wavelength. The energy difference between the ground and upper laser state is called the transition wavelength. The energy difference between an excited energy state and the upper laser state is called the quantum defect. Heat in the form of photons is emitted when electrons make a transition from an excited energy state to the upper laser state. The quantum defect results in heat generated in the gain medium. The heat produces thermal lensing and stress birefrigerence that degrades the spatial and polarization properties of the laser beam and limits the repetition ratio of the laser.
Laser shock processing utilizes a laser composed of an oscillator, amplifiers, and lenses. The amplifier contains a pump cavity with gain medium which may have the geometry of a rod, slab or other shape. The pump cavity energizes the gain medium which, in turn, produces photons. The photons are incorporated into a beam of coherent energy which traverses the gain medium.
Typically, in laser shock processing, a flashlamp is used to pump the gain medium with photons. Flashlamp pumping delivers a broad spectrum of wavelengths to the gain medium. The flashlamp energy is at wavelengths longer and shorter than the transition wavelength between the upper and ground laser states of the gain medium.
When the gain medium is pumped with photons at wavelengths shorter than the transition wavelength, the electrons of the gain medium are excited to higher energy states above the upper laser state. Consequently, a quantum defect is created between the higher energy state and the upper laser state. The relaxation of the electron from the higher energy state to the upper laser state does not produce stimulated emission. Rather, the transition from the higher energy level to the upper laser energy state results in the generation of heat. The larger the gap in energy levels, the greater the amount of heat generated. The added heat leads to thermal loading of the gain medium. Due to the laser beam energy required for laser shock processing, the amount of thermal load is substantial. Consequently, lasers traditionally used in laser shock processing include a cooling system to remove heat from the gain medium.
One disadvantage of current lasers used for laser shock processing is the pumping of the gain medium of an amplifier with photons at a wavelength shorter than the transition wavelength of the gain medium. The shorter wavelength excites gain medium electrons into higher energy states above the upper laser state. Consequently, a quantum defect exists between the two energy states. The heat generated by this energy gap produces thermal strains in the gain medium and modification of the index of refraction. For example, stress in the gain medium could result in birefringence that distorts the laser beam.
A second disadvantage of pumping the gain medium with optical energy having a photon wavelength shorter than that of the transition wavelength is the inefficient use of energy. Stimulated emission (i.e. usable photonic energy) is not produced when the gain medium electron relaxes from the higher energy states to the upper laser energy state.
Yet another disadvantage in the art of laser shock processing is a limit on the repetition rate of a laser used in laser shock processing. Due to the excessive thermal loading on the gain medium, the rate of firing of the laser is limited in order to prevent potential damage to the gain medium. A reduced firing rate provides time for heat to dissipate from the gain medium.
What is needed in the art is a laser for laser shock processing in which the laser beam amplifier is pumped with photons at a wavelength which is substantially equal to the transition wavelength of the gain medium.
What is also needed in the art of laser shock processing is a laser amplifier which is pumped in orientations that optimize the spatial profile of the laser beam for laser shock processing.
The present invention provides an apparatus for laser shock processing using a laser having an amplifier which incorporates pumping the gain medium with photons at a predetermined wavelength. By controlling the applied photon wavelength, the present apparatus minimizes the quantum defect. In addition, varying the photon wavelength and the orientation at which the photonic energy is coupled to the gain medium, the present invention can modify the spatial energy profile of the resulting laser beam.
The present invention, in one embodiment thereof, is a laser shock processing apparatus for improving properties of a workpiece by providing shock waves therein. The laser shock processing apparatus includes an energy absorbing overlay which is applied to the workpiece. The amplifier has gain medium composed of atoms having a ground state, an upper laser state, and higher energy states. A transition state energy difference is the difference in energy level between the upper laser state and the ground state. The transition state energy difference corresponds to a transition wavelength. The amplifier has an axis along which the laser beam proceeds. Pumping means is operatively coupled to the gain medium for supplying photons at a predetermined pump wavelength which minimizes the quantum defect. In a further embodiment, the pumping means includes an alexandrite laser. In an alternate embodiment, the pumping means includes a diode laser pump. In a further embodiment, there is means to adjust the diode wavelength to a desired wavelength.
The present invention, in another embodiment thereof, is a laser shock processing apparatus for improving properties of a workpiece by providing shock waves therein. The laser shock processing apparatus includes an energy absorbing overlay applied to the workpiece. A laser beam has a laser beam wavelength and spatial energy profile. An amplifier has gain medium with an axis along which the laser beam proceeds. Pumping means is operatively coupled to the gain medium for supplying photons at a predetermined pump wavelength, whereby the pumping means modifies the laser beam spatial energy profile to a desired energy profile.
The present invention, in another embodiment thereof, is a laser shock processing apparatus for improving properties of a workpiece by providing shock waves therein. The laser shock processing apparatus includes an energy absorbing overlay applied to a workpiece. An amplifier has gain medium composed of atoms having energy states. These energy states include a ground energy state, a lower laser energy state, an upper laser energy state and higher energy states. Pumping means is operatively coupled to the gain medium for supplying photons at a predetermined pump wavelength, which excites the gain medium to an excited energy state. The excited energy state is such that a difference between the excited energy state and the upper laser state is minimized.
The present invention, in another embodiment thereof, is a laser shock processing apparatus for improving properties of a workpiece by providing shock waves therein. The laser shock processing apparatus includes an energy-absorbing overlay which is applied to the workpiece. A laser beam is applied to the overlay. An amplifier has gain medium with atoms having a ground energy state and an upper laser state from which stimulated emission takes place. A diode-pumping laser is operatably coupled to the gain medium for exciting the gain medium atoms from the ground state to the upper laser state.
One advantage of the present invention is a laser amplifier which pumps the gain medium with photons at a predetermined pump wavelength which minimizes the quantum defect. By decreasing or minimizing the wavelength difference between the pump wavelength and the transition difference wavelength, thermal load on the gain medium is reduced. Consequently, undesirable thermal focusing effects are reduced or eliminated. For example, increasing the wavelength of the pumped photons to the wavelength of the transition wavelength decreases thermal stress within the gain medium which could lead to birefringence effects in the laser beam. Additionally, reduction in thermal load on the gain medium reduces possible damage to the gain medium generated when the pump wavelength is shorter than that of the transition wavelength of the gain medium.
A second advantage of the present invention is more efficient use of energy. Pumping the gain medium with photons at wavelengths near the transition wavelength allows for more efficient coupling of the pump energy to the laser beam. When the photon wavelength is shorter than the transition wavelength, the gain medium electrons are excited to higher energy levels above the upper laser state. The energy difference between the higher energy states and the upper laser state results in energy being dissipated in the form of heat. As a result, energy is wasted rather than being applied to the energy of the laser beam. Consequently, a portion of the photon""s energy is never realized as useful laser beam energy in laser shock processing.
However, pumping with photons at wavelengths substantially equal to the transition wavelength excites the gain medium directly to the upper laser state. As a result, photon energy applied to the gain medium results in a usable output laser beam when the electrons relax down to the lower laser state. Energy, in the form of heat, is significantly reduced.
Another advantage of the present invention is an apparatus with an amplifier pumped with photons which are coupled to the gain medium at a specific orientation to the laser beam in order to modify the laser beam spatial energy profile. The gain medium is pumped with photons at a predetermined axis relative to the laser beam transversing the gain medium. By varying the photon coupling orientation to the axis of the laser beam (e.g. transverse or longitudinal pumping), and supplying photons non-symmetrically to the gain medium, the present invention modifies the spatial energy profile of the resulting laser beam.
Yet another advantage of the present invention is an increase in useful life of a diode laser pump as compared with the life of a flashlamp. The average life of a flashlamp is on the order of approximately 106 to 107 laser processing cycles. The average life of a diode used in a diode laser pump is on the order of 109 laser processing cycles. Therefore, there is a 100 to 1000 fold increase in laser processing cycles realized a diode laser pump over traditional flashlamp pumping.